With the breakup and merger with parts of Conrail in 1999, CSX gained control of the trackage from the south side of the Long Bridge across the Potomac River and through Washington, DC, to the former B&O RR Benning Yard  in Maryland.  The CSX family of rail lines are drawn in blue as shown on the Railroad Map of Northcentral Virginia on the railroad history home page and as described below:
 

• Baltimore & Ohio Railroad
• Chesapeake & Ohio Railway
• Richmond, Fredericksburg & Potomac Railroad

A.  Baltimore & Ohio Railroad

Though the B&O RR's primary emphasis was westward expansion, after the Civil War the railroad began to organize and/or control a route from Harper's Ferry southward down the Shenandoah Valley.  The B&O RR gained control of the W&P RR and financially backed the charter of the Winchester & Strasburg RR (W&S RR) in 1867 which provided a connection with the Strasburg & Harrisonburg RR (S&H RR) at Strasburg.  The B&O RR  leased the S&H RR (originally part of the Manassas Gap Railroad), and in 1866 backed the charter of the Valley RR (VR) which constructed a line from Harrisonburg to Staunton by 1869, eventually reaching Lexington, Va. in 1883.  In 1896, having suffered financial difficulties, the B&O RR began to cease operation south of Strasburg.  The line south of Staunton was abandoned, the line between Harrisonburg and Staunton was eventually sold to the Chesapeake Western Railway, and the lease of the S&H RR was surrendered back to what was by then the Southern Railway.

Earlier, in 1835 the B&O RR constructed a line southward from Baltimore to Washington, DC.  The existing bridge across the Potomac River at that time could not support the weight of rail traffic.  As a result, a direct rail connection was not established with the Orange & Alexandria RR (via the Alexandria & Washington Railroad) until 1863 during the Civil War when a new bridge was constructed.  By 1870 the B&O RR had gained financial control of the O&A RR.  That same year however, the Pennsylvania RR, through a subsidiary the Baltimore & Potomac RR, gained control of the rail line across the bridge and extending to Alexandria, thereby severing the B&O RR's connection with the O&A RR and points south.

The B&O's response was to build a new branch line from their main line at Hyattsville, Md. to a point on the Potomac River across from Alexandria, Va.  A car float operation was established to ferry freight cars across the river to connect with the tracks of the Orange, Alexandria and Manassas.  This arrangement would remain in place until the turn of the century when the Potomac Yards were constructed.   In the interim, the B&O considered constructing a new crossing of the Potomac upriver from Washington.  The new line was intended to connect on the north end with the B&O RR's Georgetown Branch (Silver Spring, Md., to Georgetown, DC) and run south toward Fairfax, Va. with a connection to the O&A RR, and then continue south to Quantico to a connection with the Richmond, Fredericksburg & Potomac RR.  This line, though seriously considered and planned, was never constructed.

For additional information relating to the B&O RR, please refer to the section on the Southern Railway and its predecessor  lines 

B.  Chesapeake & Ohio Railway

In 1890, in order to be able to reach Washington, DC;  the C&O Rwy. completed a 99 year trackage rights agreement with the Southern Railway for use of the line between Orange and Alexandria (AF Tower).  The C&O Rwy. was able to reach Orange from Gordonsville via their lease of that line from the Southern Railway (successor to the Orange & Alexandria RR) which constructed the line.  The trackage rights agreement expired in 1989, was not renewed, and thereby ended the era of C&O Rwy trains passing through Manassas.  CSX  (successor to the C&O Rwy.), continues to lease the line between Orange and Gordonsville from Norfolk Southern (successor to the Southern Rwy).  The C&O Rwy. became a part of the Chessie System in 1973 and it in turn became part of CSX in 1980.

C.  Richmond, Fredericksburg & Potomac Railroad

In 1870 the Pennsylvania Railroad (PRR) assumed the construction of a previously authorized but never built railroad, the Alexandria & Fredericksburg Railway (A&F Rwy).  A connection was made with the Alexandria & Washington Railroad (A&W RR) and the Orange, Alexandria, and Manassas Railroad at Alexandria.  Then, in 1873 the PRR gained control of the rail line extending from the south side of the Long Bridge crossing of the Potomac River to Alexandria, the A&W RR.  In 1890 the A&W RR and the A&F Rwy were consolidated into the Washington Southern Railway Company (WS Rwy).  The RF&P responded by extending its line northward and connected with the PRR at Quantico.

In 1901, by agreement of all of the railroads wishing to interchange freight with each other between Washington and points south of Richmond, (the Pennsylvania RR, Atlantic Coast Line, Seaboard Air Line, Sou Rwy, RF&P RR, and the C&O Rwy), the WS Rwy and the RF&P RR were brought under the common management of the Richmond-Washington Company.  Each of the six railroads were given equal share ownership and management of the RF&P.  The RF&P was to serve each partner railroad on an equal basis.  A major rail yard, Potomac Yards, was constructed in Alexandria to handle the interchange of freight between the owning railroads.  In 1920 the RF&P RR absorbed the WS Rwy.

During the 1980's, freight traffic through the Potomac Yards diminished  as a result of the failed Penn Central merger and the bankruptcy of several other northeast railroads which led to the formation of Conrail, other eastern railroad mergers, and Amtrak's takeover of the North East Corridor.  This resulted in  the rerouting of much of the northeast freight traffic and eliminated the need for the Potomac Yards.  The yard was shutdown and torn out, and the RF&P RR became a subsidiary of CSX in 1991.